This invention relates generally to cathode ray tube (CRT) control systems and is more specifically directed to a shutdown circuit for a CRT responsive to the voltage level applied thereto to prevent the emission of hazardous X-radiation levels from the CRT.
In general, a video display utilizing a CRT includes a low voltage power supply for driving electron beam scanning circuitry and a high voltage power supply for energizing the accelerating grids of the CRT. The CRT's electron beam current is controlled by the relative potentials on the cathode and control grid electrodes with the cathode being coupled directly to the video output signal. The various electrode grids in the CRT perform functions such as brightness control, picture focusing and contrast and background selection as determined by the voltage applied to a particular electrode grid.
Typically, the ultor, or anode, high voltage accelerating potential for electron beam current is derived from a voltage developed across a winding of a horizontal output transformer in the video monitor. In transistorized deflection circuits, the high voltage is a function of such factors as the regulated B+operating voltage, component values of the resonant retrace circuit, and the internal impedance of the output transformer. The high voltage value will vary as beam current is being drawn by the CRT, the high voltage increasing relatively sharply for beam current decrements at low beam current levels.
Two operating parameters, the electron accelerating potential and the electron beam current, are particularly important in establishing CRT performance levels. For example, CRT viewing screen brightness is determined primarily by electron beam current intensity. In general, optimum video display performance requires operating at the highest possible electron accelerating potential and electron beam current.
Energetic electrons striking the face plate of the CRT produce X-radiation, thus imposing a limitation on video monitor operation and presenting a potential health hazard. If the video monitor is operated at an electron accelerating potential or an electron beam current higher for than that for which it was designed, the X-radiation emitted by the CRT may exceed a predetermined maximum safe level. Operation of the video monitor at a voltage higher than that for which it was designed may be due to any one of a variety of factors such as a transient high voltage surge, a faulty high voltage regulating component, or an improper video monitor voltage setting. Whatever the source, this over-voltage situation is made even more dangerous because of the difficulty in detecting it. Indeed, even with a dangerous increase in operating voltage, the video monitor may continue to operate satisfactorily or even with an improvement in performance due to enhanced video presentation brightness.
Various approaches have been adopted in attempting to optimize CRT operation in the video monitor while regulating electron accelerating voltage and beam current so as to conform with safety requirements generally represented by an "Isodose" curve. An "Isodose" curve represents a safe combination of high voltage and beam current so as not to produce hazardous X-radiation. The maximum safe X-radiation level recognized for a video monitor in accordance with federal regulations is 5 millirem/hour, which is represented by an aforementioned "Isodose" curve for a given CRT. The area above the "Isodose" curve represents excessive X-radiation levels while the area below the curve is within generally acceptable safety limits.
The approach generally taken for maximizing electron beam current and high voltage within safe operating levels in a CRT involves the monitoring of a signal representing one of these operating parameters in a feedback arrangement for providing a shutdown signal when a safe operating limit is exceeded. The prior art discloses many such approaches such as U.S. Pat. Nos. 4,126,816 to Willis (the voltage in a secondary winding of the horizontal output transformer is sensed), 4,213,166 to Watanabe (the voltage in a tertiary winding of the horizontal fly-back transformer is sensed), 4,287,535 to Vakil (detection of excessive current on a tertiary winding of the high voltage sweep transformer in generating a shutdown feedback signal), and 4,082,986 to Gamboa (horizontal retrace pulse width is monitored and compared with a standard pulse width). Finally, U.S. Pat. No. 3,795,767 to Waltner et al discloses a high voltage protection circuit for a television receiver in which the B boost voltage, which is a horizontally derived B+voltage generated by horizontal drive circuitry and which allegedly tracks the ultor (anode) high voltage, activates a neon light switch for grounding the video output when the ultor voltage exceeds a predetermined limit. It is alleged that, since the B boost voltage tracks the ultor potential, the B boost voltage may be used to indicate excessive ultor voltage for shutting down CRT operation. This analysis is incorrect, since the high voltage applied across the CRT may increase substantially due to any number of abnormal operating conditions, such as failure of a component in the damper circuitry, failure of a deflection yoke component, or improper horizontal deflection pulse width regulation, while failing to produce a corresponding increase in the B boost voltage.
The present invention is intended to overcome these and other limitations of high voltage shutdown systems used in prior art video monitors. The present invention does not rely upon an output which indirectly reflects the high voltage value, but rather senses the original high voltage input which drives the high voltage sweep transformer in more accurately monitoring and feeding back a signal reflecting the voltage applied across the CRT.